1. Technical Field
This invention generally relates to heat exchangers for extracting the heat from processing exhausts, and more particularly, this invention relates to an apparatus and method for extracting heat from the contaminated steam produced by food processing, especially french fry fryers and the like, and for reducing pollution in both the exhaust vented to the atmosphere and in the liquid effluent produced.
2. Background Art
Food processing plants consume vast amounts of energy, primarily in the form of steam used for steam peeling, blanching, cooking, etc., and for heating water, a substantial amount of which is used for keeping everything in the plant clean. Upwards of 20% of the energy used in a french fry processing plant is used to heat potable water for cleaning and washing.
Industrial french fry fryers are large, long, continuous feed fryers which can fry upwards of 25,000 pounds per hour of potatoes during the processing operation. The oil in which the potatoes are fried is generally held at 380.degree. F. and the raw french fries are continuously fed into this oil. During the cooking process, approximately 25% of the moisture content of the potatoes is removed. The hot oil boils the moisture from the potato pieces and the water leaves the oil as bubbles of steam. Being lighter than air, this steam continues to rise upward. Generally a hood above the oil confines the steam and allows it to escape or be drawn upward through a vent stack to atmosphere. Assuming a raw potato feed rate of 25,000 lbs. per hour, this translates to between 5,000 to 7,000 pounds per hour of steam being wasted by venting to the atmosphere. This steam has entrained within it organic hydrocarbons, trace minerals and frying oil. Since the steam is contaminated, it has always been considered pure waste. The mixture of steam, oil, hydrocarbons and trace minerals is later referred to in this disclosure as waste gas.
As previously stated, the waste gas released by the fryer and hood is actually not pure steam, but rather a mixture of steam, air and cooking fumes. The fumes consist of suspended droplets of oil and organic hydrocarbon gasses. Some of the organic gasses can be condensed at lower (cooling water) temperatures and some of these gasses cannot be readily condensed. In addition, air, which is also not condensable, is drawn into the fryer's hood from outside of the fryer by the draft created when the steam rises into the vent stack. Many of these vent stacks release the waste gas directly to the atmosphere. Some fryers are equipped with vent fans which enhance or power the flow of exhaust gas leaving the fryer. Some fryers use cold or cool water in direct contact with the waste gas stream, often in spray chambers after the fryer, so that the oil droplets, condensable cooking fumes and the steam with its heat energy are condensed or "scrubbed" from the non-condensable gasses and are carried away with the sprayed water as liquid waste. Fryers not having wet scrubbers waste the steam and cooking fumes to the atmosphere causing air pollution.
A heat exchanger could be used to condense the steam and heat water for other uses, but the problem with applying conventional condensing heat transfer technology to extract the heat from the waste stream is that as the steam condenses the air and other non-condensable gases concentrate or collect next to the heat transfer surfaces and form a barrier. This barrier is an insulator, which greatly impedes the heat transfer from the steam to the cooling water. If the non-condensables are not continuously removed from the condenser, they will eventually fill it and all heat transfer between the steam and cooling water will stop.
DRAWERT ET AL., U.S. Pat. No. 4,426,210 discloses a process for washing the odor-emitting substances from waste air which incorporates an air to liquid heat exchanger, a gas washing chamber and a vent fan to pull the waste air through the apparatus. Unfortunately, the device is not designed to handle large quantities of steam or other condensable vapors or to remove condensate and liquid or condensed pollutants, such as oil and the like. It is readily apparent that the heat exchanger of this invention would quickly become fouled by the water and oil which is necessarily extracted in the heat exchanger.
CHILDS, U.S. Pat. No. 4,708,722 teaches a combination air washer and liquid to liquid heat exchanger for removing heat and pollutants from the smoke present in a chimney or the like. Unfortunately, the only significant condensing gas to liquid heat transfer which takes place in this apparatus happens in the washing process by the water droplets as they fall toward the collection basin. Obviously, this is unacceptably inefficient for recovering the vast amount of heat generated by a typical industrial french fryer or similar cooker. Since the waste gas is first directly contacted and mixed with the water stream from which heat is recovered, that water stream must be held at a low temperature or much of the heat of the waste gas will exit with the exhaust as vaporized water. If the water collected at the bottom of the spray chamber is substantially below boiling temperature, then the heat recovered by the heat exchanger will be at an even lower temperature and therefore of less use.
Both of the above prior art devices cited require and use an external source of water which directly contacts the waste gas and produces a dilute liquid waste stream which is substantially cooled by the supplied water.
The inventor is unaware of any prior solution which is tailored to the peculiar problems presented by the waste steam produced by large food processing fryers and the like. Two of the primary problems presented by this application are the provision for constant removal of condensate and other agglomerates from the surfaces of the gas to liquid heat exchanger and a method of separating the oil from both the exhaust gas and the condensed water. What is needed is an apparatus and method for recovering heat from contaminated waste steam, which doesn't require continual manual cleaning and maintenance and which will reduce the pollutants vented to the atmosphere and in the liquid effluent produced.
It is therefore an object of this invention to provide a means for extracting the heat from the waste gases of the big commercial frying machines by condensing the waste steam in a down flow cooled surface heat exchanger, and using the extracted heat for other purposes such as heating potable water for plant cleaning and the like. It is a further object of this invention to provide a means for removing a substantial amount of the oil and hydrocarbons present in both the waste gas and liquid effluent produced by industrial fryers and the like.